SAN FRANCISCO — Still under pressure to reduce the number of inmates in its crowded prisons, California has steadily increased the number of convicts it sends to private institutions outside the state since Gov. Arnold Schwarzenegger began the program in 2006.

The latest deal will see 5,800 inmates sent to private prisons across state lines, bringing the total to more than 15,000, according to reports. The transfers will begin in May under a contract that runs through June 2013. With a prison population of about 164,000, the state only has corrections facilities equipped to house around 100,000.

Critics says moving prisoners to out-of-state facilities does little to relieve the underlying problems causing crowded conditions and questioned the timing of the new, no-bid contracts with two private companies – The GEO Group and Corrections Corp. of America.

When California signed contracts to ship prisoners over state lines four years ago, it began with 2,260 inmates at a cost of $51 million annually. Currently, it is set to pay the companies $360 million a year to house 15,424 prisoners, and spend more than $636 million annually once administrative costs are factored in.